The present invention relates in general to a process for treating aqueous waste. More particularly, this process involves treating a copper waste stream resulting from chemical mechanical planarization (CMP).
Chemical mechanical planarization (CMP) is a new method of applying copper to semiconductors. In fact, using copper to make semiconductors is new to the industry. CMP is a process used by the semiconductor industry to produce very smooth surfaces on each layer of a microchip by "sanding" the microchip with a water slurry of silica or alumina. In making microchips for computers, pagers, phones and other electronic equipment, a large volume of purified water is used for the dilution and rinsing steps of the CMP process. When copper layers are planarized, the waste water from the process contains relatively large amounts of dissolved copper.
Currently, because CMP is a new process, copper waste is merely drummed and shipped off. Similar wastewater created in other industries, such as the plating industry or the printed circuit board industry, generally is treated in an electrowinning process to remove copper. The disadvantage with such a process is that, because particles suspended in the copper CMP waste water are not removed before the electrowinning process, the separated copper is impure. Another disadvantage with electrowinning the copper CMP waste water is that the concentration of copper may be too low for effective electrowinning.
Other methods for removing metals from waste water include precipitation of the metals as their oxides, as is well known in the art. The precipitated metals are removed from the water by filtration methods such as bag filtration, ultrafiltration or microfiltration. However, such methods suffer from the shortcomings that they can produce a hazardous sludge, which has no value, and that some metals are not precipitated and therefore remain in solution. In fact, disposing of hazardous sludge costs more than disposing of typical sludge due to its hazardous nature.
Further methods of removing metals from waste water include using solids de-watering devices such as filter presses, belt presses, or centrifuges to concentrate the solids. However, solids are difficult to remove from these devices, and the operation of these devices is labor intensive. While solids de-watering devices exist which operate automatically and discharge solids automatically, these devices are too expensive to be viable options for many.
Thus, a process for treating waste water containing copper CMP particles is needed which can overcome the above disadvantages. Specifically, a process is needed that will remove copper from the solids and liquids in the waste water without the use of a solids de-watering device. Furthermore, a process which does not produce a hazardous sludge is desired. In addition, a process which recovers the copper in a form which has value, such as metallic copper, would be preferred.